This invention relates generally to a multi-pressure compensation system, apparatus and method for variable displacement pumps. In a particular aspect of the invention, a pressure control port of a variable displacement hydraulic pump is controlled so that the pump pumps a fluid at pressure and flow rate combinations within the range between a maximum pressure, minimum flow rate combination and a minimum pressure, maximum flow rate combination without exceeding the maximum power output of a prime mover driving the pump, which maximum power output is less than the power which would be needed for the pump to pump the fluid at a maximum pressure, maximum flow rate combination.
Fluid energized equipment such as can be used in the oil and gas industry, for example, is commonly connected to and energized by a single power pack containing a variable displacement pump which pumps the fluid to energize the equipment. The power pack also includes a prime mover, such as a diesel engine, which drives the pump. The hydraulically actuated equipment is typically connected in parallel to the variable displacement pump. Each equipment is usually connected through a throttling valve or electrohydrauic valve which controls the pressure applied to the equipment, thereby regulating the speed of the equipment. An illustration of this arrangement is shown in FIG. 1 wherein the fluid energized equipment is shown as loads 2.sub.1, 2.sub.2, . . . 2.sub.n connected through throttling or electro-hydraulic valves 4.sub.1, 4.sub.2, . . . 4.sub.n, respectively, to a hydraulic power pack 6. By way of example, the loads 2 might include hydraulically driven motors, metering pumps and sand screws.
The loads 2 typically require a higher pressure to get started than they need to continue running once started. For example, a starting pressure might be 2,500 pounds per square inch (psi) and a running pressure might be 1,000 psi. At the starting pressure a relatively low fluid flow rate is typically needed, but at running pressure a relatively high fluid flow rate is typically needed.
The pressure applied to a particular load 2 is controlled by the respective valve 4, but the primary system pressure is delivered by the variable displacement pump of the power pack 6. When the primary system pressure is greater than the pressure applied to a load, there is a pressure drop across the respective valve 4. Such a pressure drop generates excess heat which is wasted energy. Normally, however, the variable displacement pump must be set for the worst case pressure requirement, i.e., high start-up pressure to start the loads. Therefore, after start-up, the variable displacement pump provides more pressure than is needed and the excess pressure is converted to the aforementioned excess heat. This excess heat puts an additional load on the power pack cooling system.
The typical variable displacement pump used in the above-described system has been operated at a limited operating range so that once set for the worst case pressure requirement, it could not vary much to reduce the system pressure and thereby reduce the throttling valve pressure drop and resultant excess heating. That is, such a pump typically has been set to accommodate the high pressure, low flow rate combination of operating conditions needed for load start-up; but has not been controlled to provide low pressure, high flow rate combinations of operating conditions once the loads have come on line and been started. The pump could be preset to provide low pressure and high flow rates, but then it would not accommodate the start-up conditions (an example of the operating range for such a limited setting is shown in FIG. 5 by the lines 7 of short dashes). Current applications in at least the oil and gas industry need more flexibility than this type of operating range limited system can provide.
One way to improve the operating range of the variable displacement pump would be to use a larger prime mover which can provide "corner power," i.e., sufficient power to drive the pump to provide maximum pressure and maximum flow rate. This, however, requires a heavier and costlier prime mover which cannot always be accommodated. Furthermore, this larger capacity is not needed at any one setting of the variable displacement pump, but is only needed to provide the overall expanded operating range.
Another way to try to improve the operating range of the variable displacement pump driven by a power limited prime mover is with a mechanical "horsepower compensation" spool from Parker Hydraulics. Testing was conducted on this, but it did not provide for constant output power and it was not adjustable over a wide enough range. A graph of an example of the flow versus pressure characteristics of the compensation by the Parker horsepower compensation spool is shown by line 8 in FIG. 5.
Thus, there is the need for an apparatus which can control a variable displacement pump to operate over a wider operating range of pressure and flow rate combinations even when the pump is driven by a power limited prime mover having a maximum power output below the "corner power." There is the need for an overall pumping system and method within which a variable displacement pump provides fluid flow at pressure and flow rate combinations throughout a range between maximum pressure, minimum flow rate and minimum pressure, maximum flow rate without requiring the prime mover to have enough power to drive the pump simultaneously at maximum pressure and maximum flow rate.